It is previously known to change the properties of wood products by pressure treatment. Pressure treatment has been used, for example, for compressing and hardening of wood. In this connection, particularly good results have been obtained by treatment by means of isostatic pressurization of the wood elements. In a previously known method, the wood elements to be treated are placed surrounded by a pressure medium in a compression chamber. The pressure medium consists of a plurality of adapted rubber elements, shaped, for example, as balls, elongated strips or cubes. The pressure medium is delimited in the pressure chamber from a working fluid, for example hydraulic oil, by an elastic membrane. By pressurizing the working fluid by means of a hydraulic pump, the worked-up pressure is transmitted to the pressure medium. The pressure medium forms around the wood elements and brings about a uniform compression thereof. This results in a permanent compression and hardening of the wood elements.
A disadvantage with the prior art is that the liquid and moisture contents of the wood elements prior to the pressure treatment must be reduced to a level acceptable for the pressure treatment. The reason therefor is that the incompressible liquid during the pressurization is contained in the wood element, whereby compression of the wood element is not possible. Thus, it has not been possible to pressure-treat freshly sawn timber or other wood products with too high a moisture ratio.
A closely related problem is that, with the previously known technique, it has not been possible to utilize pressure treatment for the very purpose of drying of wood elements. To reduce the moisture ratio of wood products, it has hitherto been necessary to use the traditional methods, which are based on heating and/or air drying by means of fans. These methods, however, are relatively time-consuming and therefore cause high costs.
Another, and perhaps even more serious problem, which is a consequence of the traditional drying methods, concerns the subsequent impregnation of the dried wood products. This often entails serious problems, since it is difficult to cause the impregnating agent to penetrate sufficiently deep into the wood. Impregnation of wood products, such as sawn timber, is often desirable. The impregnation aims at increasing the resistance of the wood products to certain processes, such as bacterial or fungus attack, causing degradation in the wood. Usually, the preserving agent is dissolved in a liquid, which by means of various methods is brought to penetrate into the wood. The penetration may be achieved, for example, by soaking the wood products or by driving in the impregnating liquid by means of an over-pressure. In the latter case, the impregnation is usually preceded by vacuum treatment of the wood products.
The penetration of the liquid into the wood may take place either by diffusion or flow. In the case of diffusion, the liquid penetrates very slowly into the wood by means of the concentration of the impregnating solution. In the case of penetration by flow, on the other hand, the liquid may quite rapidly penetrate into the wood by utilizing the fibres and pores occurring in the wood. During impregnation, flow penetration is preferable to diffusion penetration owing to the higher rate of penetration.
In coniferous wood, more than 90% of the wood consists of wood fibre, so-called tracheids. In the live tree, the purpose thereof is, among other things, to conduct liquid. The tracheids consist of about 3 millimetres long, elongated hollow fibres. They are arranged essentially parallel to the longitudinal direction of the tree and each other and are mutually axially displaced. Liquid may be transported from one tracheid to an adjacent one via so-called pores. The pores, which may be of different kind, for example ring pores or simple pores, constitute openings in the tracheid wall. The pores usually comprise some type of closing member, a so-called pore membrane. Because the pore membranes open and close the pores, liquid is allowed and prevented, respectively, from passing from one tracheid to another.
During impregnation of sawn timber, the liquid penetrates very rapidly from the end surfaces of the wood elements. The longitudinal tracheids are cut off there and the liquid enters easily. To allow the liquid to pass into the wood, from one tracheid to another, the pores must be open. Sooner or later the liquid encounters a tracheid where all the pores are closed and the penetration thus stops.
It has proved that traditional drying of coniferous wood causes closing of the pores. When the wood dries, the pore membrane is displaced from a central position and closes the pore opening. What causes the membrane to move are capillary forces in the water which is dried away. When the membrane has clogged the pore opening, it is impossible to move the membrane even if the wood is subjected to very high pressure. This is probably due to the membrane adhering to the pore wall and to the fact that a bond in the form of hydrogen bridges arises therebetween.
The above reasoning is an explanation why, after traditional drying of softwood, it is so difficult to cause impregnating liquid to penetrate sufficiently deeply into the wood. Further, it has been known for a long time that it is considerably more difficult to impregnate spruce than pine. This is due, among other things, to the fact that a larger number of pores close during drying of spruce than of pine, and to pine having fewer and smaller pores.
A special problem with previously known drying methods is thus that they render the subsequent impregnation of the wood considerably more difficult. This is particularly true of certain kinds of wood, such as spruce.
The object of the present invention is to provide a method for treatment of wood which allows pressure treatment to be used for the drying of the wood and which makes possible a considerably simplified impregnation of the dried wood.